Method for making micromechanical structures having at least one lateral, small gap therebetween and micromechanical device produced thereby

ABSTRACT

A method and resulting formed device are disclosed wherein the method combines polysilicon surface-micromachining with metal electroplating technology to achieve a capacitively-driven, lateral micromechanical resonator with submicron electrode-to-resonator capacitor gaps. Briefly, surface-micromachining is used to achieve the structural material for a resonator, while conformal metal-plating is used to implement capacitive transducer electrodes. This technology makes possible a variety of new resonator configurations, including disk resonators and lateral clamped-clamped and free-free flexural resonators, all with significant frequency and Q advantages over vertical resonators. In addition, this technology introduces metal electrodes, which greatly reduces the series resistance in electrode interconnects, thus, minimizing Q-loading effects while increasing the power handling ability of micromechanical resonators.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 09/938,411 filedAug. 23, 2003, entitled “Method For Making Micromechanical StructuresHaving At Least One Lateral, Small Gap Therebetween And MicromechanicalDevice Produced Thereby” which claims the benefit of U.S. provisionalpatent application Ser. No. 60/227,507 filed Aug. 24, 2000 and entitled“Process Technology For Lateral Small-Gap Micromechanical Structures”.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under DARPA Contract No.F30602-97-2-0101. The Government has certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to methods of making micromechanical structureshaving at least one lateral, small gap therebetween and micromechanicaldevices produced thereby.

2. Background Art

Vibrating mechanical tank components, such as crystal and SAWresonators, are widely used for frequency selection in communicationsub-systems because of their high quality factor (Q's in tens ofthousands) and exceptional stability against thermal variations andaging. In particular, the majority of heterodyning communicationtransceivers rely heavily on the high Q of SAW and bulk acousticmechanical resonators to achieve adequate frequency selection in RF andIF filtering stages and to realize the required low phase noise andstability in their local oscillators. In addition, discrete inductorsand variable capacitors are used to properly tune and couple the frontend sense and power amplifiers, and to implement widely tunablevoltage-controlled oscillators.

At present, the aforementioned resonators and discrete elements areoff-chip components, and must interface with integrated electronics atboard level, often consuming a sizable portion of the total sub-systemarea. In this respect, these devices pose an important bottleneckagainst the ultimate miniaturization and portability of wirelesstransceivers. For this reason, many research efforts have been focusedon strategies for either miniaturizing these components or eliminatingthe need for them altogether.

The rapid growth of IC-compatible micromachining technologies that yieldmicro-scale, high-Q tank components may now bring the first of the abovestrategies closer to reality. Specifically, the high-Q RF and IFfilters, oscillators, and couplers, currently implemented via off-chipresonators and discrete passives may now potentially be realized on themicro-scale using micromachined equivalents based on a variety of noveldevices, including high-Q, on-chip, vibrating mechanical resonators,voltage-tunable, on-chip capacitors, isolated, low-loss inductors,microwave/mm-wave medium-Q filters, structures for high frequencyisolation packaging, and low-loss mechanical switches. Once theseminiaturized filters and oscillators become available, the fundamentalbases on which communication systems are developed may also evolve,giving rise to new system architectures with possible power andbandwidth efficiency advantages.

Prototype high-Q oscillators featuring lateral comb-drivenmicromechanical resonators integrated together with sustainingelectronics, all in a single chip, using a planar process that combinessurface-micromachining and integrated circuits, have been demonstrated.The gap between the electrodes and the structure of the comb-drivenmicromechanical resonator is limited by lithography capability.Therefore, a submicron gap is very difficult to do. As the frequency ofthe resonator goes higher, the size of the resonator becomes smaller. Sothe electromechanical coupling is smaller. In order to increase theelectromechanical coupling, a small-gap between the electrode and thestructure is necessary. Although the capacitive gap of verticalmicromechanical resonators, which is defined by the thickness of asacrificial layer, can be very small, clamped-clamped beam verticalmicromechanical resonators suffer from lower Q due to anchordissipation. Also, it normally has only one port which limits itsapplication range. Lateral resonators, on the other hand, haveadvantages of greater geometric design flexibility and more ports thannormally attainable via vertical resonators. However, theelectrode-to-resonator gap for capacitively-driven lateral resonatorshas historically been implemented via lithography and etching, and thisgreatly limits the degree by which the electrode-to-resonator gapspacing can be reduced.

In order to increase the electromechanical coupling for a lateralmicromechanical resonator, a process to form a lateral submicron gapbetween an electrode and the resonator structure, without the need foradvanced lithography tools, is desired.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an improved method ofmaking micromechanical structures having at least one lateral, small gaptherebetween and a micromechanical device produced thereby.

In carrying out the above object and other objects of the presentinvention, a method is provided for making micromechanical structureshaving at least one lateral gap therebetween. The method includesproviding a substrate, and surface micromachining the substrate to forma first micromechanical structure having a first vertical sidewall and asacrificial spacer layer on the first vertical sidewall. The method alsoincludes forming a second micromechanical structure on the substrate.The second micromechanical structure includes a second vertical sidewallseparated from the first vertical sidewall by the spacer layer. Themethod further includes removing the spacer layer to form a firstlateral gap between the first and second micromechanical structures.

The step of surface micromachining may further form a third verticalsidewall on the first micromechanical structure with the sacrificialspacer layer thereon and the method may further include forming a thirdmicromechanical structure including a fourth vertical sidewall separatedfrom the third vertical sidewall by the spacer layer. The step ofremoving may further form a second lateral gap between the first andthird micromechanical structures.

The second micromechanical structure may include an electrode. The firstmicromechanical structure may include a resonator wherein the firstlateral gap is an electrode-to-resonator capacitive gap.

The step of forming may include the step of plating metal on thesubstrate wherein the second micromechanical structure is a plated metalelectrode.

The step of forming may include the step of selective epitaxial growth(SEG) to define the second micromechanical structure.

The method may further include preventing metal from being plated on thefirst micromechanical structure.

The first lateral gap is preferably a submicron gap.

Further in carrying out the above objects and other objects of thepresent invention, a micromechanical device is provided. The deviceincludes a substrate, a first micromechanical structure supported on thesubstrate and having a first vertical sidewall, and a secondmicromechanical structure supported on the substrate and having a secondvertical sidewall. The device further includes a first submicron lateralgap between the first and second vertical sidewalls to increaseelectromechanical coupling of the first and second micromechanicalstructures.

The second micromechanical structure may be a plated metal electrode oran SEG grown electrode and the first micromechanical structure may be alateral resonator.

The first micromechanical structure may have a third vertical sidewalland the device may further include a third micromechanical structuresupported on the substrate and having a fourth vertical sidewall and asecond submicron lateral gap between the third and fourth verticalsidewalls to increase electromechanical coupling of the first and thirdmicromechanical structures.

The lateral resonator may be a polysilicon resonator such as aflexural-mode resonator beam.

The substrate may be a semiconductor substrate such as a siliconsubstrate.

The first submicron lateral gap may be a capacitive gap.

The second and third micromechanical structures may be electrodes suchas plated metal electrodes.

The first and second submicron lateral gaps may be capacitive gaps.

The above object and other objects, features, and advantages of thepresent invention are readily apparent from the following detaileddescription of the best mode for carrying out the invention when takenin connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a side sectional schematic view of an insulation layercomprising oxide and nitride layers formed on a substrate, a patternedpolysilicon layer and a sacrificial oxide layer deposited thereon;

FIG. 1 b is a side sectional schematic view of the layers of FIG. 1 aafter opening for anchors are formed and a patterned polysilicon layerand a gap sacrificial oxide deposited thereon;

FIG. 1 c is a side sectional schematic view of the sacrificial oxideafter etching and an evaporated seed layer together with the structuresof FIG. 1 b;

FIG. 1 d is a side sectional schematic view of a thick photoresist forplanarization etch back which has been spun on the structures of FIG. 1c;

FIG. 1 e is a side sectional schematic view with the PR etched back tothe top of the structures and the seed layer etched on the top of thestructures;

FIG. 1 f is a side sectional schematic view of the structures of FIG. 1e after the PR is stripped, a PR plating mold is formed and Auelectrodes are plated;

FIG. 1 g is a side sectional schematic view of the structures of FIG. 1f with the PR mold stripped, the seed layers removed and an Ni layerformed on the electrodes; and

FIG. 1 h is a side sectional schematic view of the structures of FIG. 1g after HF release and the Ni layer removed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment for a small-gap, lateral resonator process flowof the present invention is presented in FIGS. 1 a-1 h. As shown in FIG.1 a, this process starts with a 2 μm thick oxide film 10 (i.e. SiO₂)thermally grown on a silicon substrate 12 and a 3000 Å thick film 14 ofnitride (i.e. Si₃N₄) which together serve as an isolation layer. After a3000 Å thick low stress polysilicon layer 16 is deposited via LPCVD,doped and patterned via reactive ion etching (RIE), a 5000 Å thick layer18 of sacrificial oxide (i.e. SiO₂) is deposited by LPCVD.

As shown in FIG. 1 b, vias 20 are patterned into the sacrificial oxidelayer 18 by RIE, exposing the underlying polysilicon layer 16 inspecific areas to later serve as anchors for eventual structures. A 2 μmthick structural layer of low stress polysilicon is then deposited viaLPCVD and patterned also via RIE to form anchor structures 22 and aresonator structure 23 with straight side walls. A 1000 Å thick layer 24of conformal LPCVD oxide is then deposited in order to define thesmall-gap spacing of the present invention. This oxide could also bethermally grown over the polysilicon or silicon structures.

As shown in FIG. 1 c, the sacrificial oxide layer 18 is then etched (RIEand wet etch) until the isolation nitride layer 14 is exposed in regionswhere metal electrodes are to be formed. A thin metal layer (Cr200Å/Au300 Å/Cr200 Å) is then evaporated over all areas of the wafer toserve as a seed layer 26 for electrode plating. The top Cr layer of theseed layer 26 is used to enhance the adhesion between the seed layer 26and a plating mold while the bottom Cr layer of the seed layer 26 is forthe adhesion between the middle Au layer and the underneath nitridelayer 14.

As shown in FIG. 1 d, in order not to plate metal on top of thestructures 22 and 23 while forming the electrode, a thick layer 28 ofphotoresist (PR) is first spun on. Then, the layer 28 is planarized andetched back via RIE to expose the seed metal layer 26 on top of thestructures 22 and 23 as shown in FIG. 1 e. The seed layer 26 on top ofthe structures 22 and 23 is then removed by wet etching to prevent metalfrom plating over the tops of the polysilicon structures 22 and 23during subsequent electroplating steps.

As shown in FIG. 1 f, after the rest of the PR is removed, a plating PRmold 30 is formed by lithography, the Cr layer on top of the exposedseed layer 26 is removed and then Au electrodes 32 are plated on theexposed Au layer of the seed layer 26 between vertical side walls of theresonator structure 23 and the photoresist mold 30 which together definethe electrode plating boundaries.

As shown in FIG. 1 g, a thin layer 34 of Ni is plated on the electrodes32 in order to protect the surface of the Au electrode regions whileportions of the seed layer 26 are being removed.

FIG. 1 g shows the PR mold 30 and the portions of the seed layer 26removed.

As shown in FIG. 1 h, the layer 34 of Ni is removed and finally, theresonator structure 23, separated by sub-micron gaps 38 between the twometal electrodes 32, is free to move after HF release to remove thelayer 24 and the layer 18.

Benefits accruing to the invention are numerous. For example, the mainadvantages and contributions of this invention are:

-   -   (i) metal electrodes: less interconnect resistance, more power        handling;    -   (ii) submicron, sacrificial-film-determined lateral gaps between        the resonator and the electrodes;    -   (iii) higher Q in some resonators, given the anchoring options;    -   (iv) increase electromechanical couplings, thus increase the        efficiency of resonators, gyroscopes, accelerometers, etc.;    -   (v) allow more flexible mechanical circuit configurations;    -   (vi) make high frequency disk resonators possible;    -   (vii) makes stress-compensated resonators possible; and    -   (viii) makes two-port resonator oscillator configurations easier        to manufacture.

The method of the invention can be used to form:

(1) micromechanical structures (including resonators, gyroscopes, andaccelerometers, etc.) driven and sensed by metal electrodes plated alongthe side walls of the structure; and

(2) small capacitive gaps between the micromechanical structure andplated metal electrodes defined by the thickness of sacrificial layer(not only oxide, this sacrificial layer can be any kind of material).

The etch back process used to prevent metal plated on top of theresonator structure 23 (FIGS. 1 d-1 e) is also particularly useful. Alsoparticularly useful is the seed layer combination Cr/Au/Cr or Cr/Ni thatsurvives in straight HF release. Optional plated metals (Au, Ni, Pd, Pt,Cu) also can serve as electrode materials. Alternatively, the processcan be modified wherein epi-Si is grown to serve as the electrodes.

While embodiments of the invention have been illustrated and described,it is not intended that these embodiments illustrate and describe allpossible forms of the invention. Rather, the words used in thespecification are words of description rather than limitation, and it isunderstood that various changes may be made without departing from thespirit and scope of the invention.

1. A method for making micromechanical structures having at least onelateral gap therebetween, the method comprising: providing a substrate;surface micromachining the substrate to form a capacitively-driven,lateral micromechanical structure having a first vertical sidewall and asacrificial spacer layer on the first vertical sidewall; forming a firstcapacitive transducer electrode on the substrate, the first capacitivetransducer electrode including a second vertical sidewall separated fromthe first vertical sidewall by the spacer layer; and removing the spacerlayer to form a first lateral submicron capacitive gap between themicromechanical structure and the first capacitive transducer electrodeto increase electromechanical coupling therebetween.
 2. The method asclaimed in claim 1 wherein the step of surface micromachining furtherforms a third vertical sidewall on the micromechanical structure withthe sacrificial spacer layer thereon and wherein the method furthercomprises forming a second capacitive transducer electrode including afourth vertical sidewall separated from the third vertical sidewall bythe spacer layer and wherein the step of removing further forms a secondlateral submicron gap between the micromechanical structure and thesecond capacitive transducer electrode.
 3. The method as claimed inclaim 1 wherein the micromechanical structure includes a resonator andwherein the first lateral submicron capacitive gap is anelectrode-to-resonator capacitive gap.
 4. The method as claimed in claim1 wherein the step of forming includes the step of plating metal on thesubstrate and wherein the first capacitive transducer electrode is aplated metal electrode.
 5. The method as claimed in claim 4 furthercomprising preventing metal from being plated on the micromechanicalstructure.
 6. The method as claimed in claim 1 wherein the step offorming includes the step of growing the first capacitive transducerelectrode via selective epitaxial growth.
 7. The method as claimed inclaim 1 wherein the step of forming includes the steps of depositingpolysilicon and etching the polysilicon to form the first capacitivetransducer electrode.